1. Field of the Invention
The present invention relates to a substrate treatment process which treats a long substrate continuously by a roll-to-roll system, or a like system. In particular, the present invention relates to a substrate treatment process which comprises plural treatment steps in which the substrate is delivered with application of a tensile force thereto. The present invention relates further to a process for forming a photovoltaic element, so-called solar cell.
2. Related Background Art
In recent years, various studies are being made for practical solar power generation by a solar cell. For practical use of the solar cell for power supply, the solar cell should have sufficiently high photoelectric conversion efficiency and high reliability, and should be suitable for mass production.
Solar cells employing amorphous silicon are attracting attention because of possibility of mass-production at a low cost (hereinafter the amorphous silicon is referred to simply as xe2x80x9ca-Sixe2x80x9d). This is because, in the production of the a-Si solar cell, a deposition film of a semiconductor or the like can be formed from a readily available source gas like a silane by glow discharge decomposition on an inexpensive belt-shaped substrate such as metal sheets or resin sheets. Various methods are disclosed for the process and apparatus for the a-si solar cell.
FIG. 1 is a schematic sectional view of an a-Si solar cell. In FIG. 1, the solar cell is constituted of a substrate 101, back reflection layers 102-103, semiconductor layers 104-106, a transparent electroconductive layer 107, and collecting electrodes 108. Hereinafter the assembly of the parts 101-107 is called a solar cell slab (or simply, a slab).
In a production process of an a-Si solar cell, substrate 101 is a belt-shaped electroconductive substrate of stainless steel or the like wound in a rolled state; back reflection layer 102-103 is constituted of metal layer 102 formed from highly reflective silver (Ag), aluminum (Al), copper (Cu), or the like, and transparent oxide layer 103 having a suitable resistance formed from zinc oxide (ZnO), tin oxide (SnO2), or the like between the metal layer and the semiconductor layer. These layers are formed by means of a roll-to-roll type of continuous sputtering apparatus as disclosed, for example, in Japanese Patent Application Laid-Open No. 6-184745.
As a semiconductor film formation apparatus, a roll-to-roll type of continuous plasma CVD apparatus is disclosed in U.S. Pat. No. 4,485,125. An n-Type semiconductor layer 104 and p-type semiconductor layer 106 can be formed by an RF plasma CVD process (RFPCVD process). An i-Type semiconductor layer 105 can be formed by an RF plasma CVD process, or a microwave plasma CVD process (MWPCVD process) with a roll-to-roll type apparatus disclosed in Japanese Patent Application Laid-Open No. 3-30419. The microwave having a high frequency can give a higher energy density than RF, and is suitable for plasma generation and maintenance thereof at a low pressure. These processes are capable of forming high-quality deposition film by preventing polymerization of active species not to deteriorate the deposition film, and preventing generation of dust of polysilanes or the like in the plasma to improve dramatically the film formation speed.
Transparent electroconductive layer 107 can be formed from an electroconductive material transparent to visible light such as SnO2, In2O3 and ITO (In2O3+SnO2) by a roll-to-roll type of continuous sputtering apparatus.
After deposition of the transparent electroconductive layer, the rolled substrate is cut into slabs in a prescribed size. Thereon, collecting electrodes 108 are formed by soldering the wire of copper, silver, or the like in the solar cell module process line.
In mass production of solar cells by the roll-to-roll system, the belt-shaped substrate is preferably made longer and broader, and is preferably made thinner for lower production cost.
In the belt-shaped electroconductive substrate production process, the broad substrate is slit in the length direction into a prescribed width in a slitter process line to obtain the substrate product for layer formation. This slitting process causes waving at the cut edges of the slitted belt-shaped substrate (hereafter this waving referred to as xe2x80x9cedge wavinessxe2x80x9d).
The production of a solar cell by a roll-to-roll system comprises essentially plural treatment steps for cleaning, a back reflection layer formation, a semiconductor layer formation, and so forth. In each of the steps, the belt-shaped substrate is wound off from a roll, delivered, and wound up by another roll. The above steps are generally different in the treatment conditions such as in the length of the delivery path, inclination in the delivery direction, the state of the delivery path (linear, curved, folded, etc.), the atmosphere of the treatment, the substrate temperature, and tensile force applied to the substrate.
In a production process of a solar cell, for example, the substrate is cleaned in a liquid at an atmospheric pressure at a temperature ranging from room temperature to 100xc2x0 C., and the back reflection layer and the semiconductor layer are formed in a vacuum respectively at a different substrate temperature. The difference in the substrate treatment conditions during the substrate delivery may cause deformation of the substrate: deformation by stress, deformation by heat from a heater or the plasma, deformation by stress in deposition film of the deposition layer or by external stress given by steering mechanism of a roll-to-roll treatment apparatus. Such deformation of the substrate tends to become remarkable with every passage through the treatment steps, which may enlarge the edge waviness.
Although the edge waviness can be reduced during the treatment to some extent by increasing the tensile force in the treatment apparatus, the edge waviness will become enlarged after the treatment.
In the aforementioned roll-to-roll type of continuous plasma CVD apparatus, the occurrence of the edge waviness by the substrate deformation may cause contact of the deformed portion with the aperture-adjusting plate to cause generation of dust and to deteriorate the properties of the formed film, or to cause scratching of the film face to destroy the semiconductor bonding element at the scratched portion. Further, this substrate is delivered so as to serve as a lid of a discharge box. Therefore, the occurrence of the edge waviness of the substrate may cause leakage of plasma from the discharge chamber to render the discharge unstable, or to cause interruption of the discharge, or to impair uniformity of the film properties, which may lower the production yield.
The present invention intends to provide a substrate treatment process which solves the aforementioned problems, and which is capable of forming continuously a deposition film with a uniform property and uniform thickness with stable discharge at a high production yield.
The substrate treatment process of the present invention is constituted as shown in the items (1)-(7) below to solve the above problems:
(1) A substrate treatment process comprising plural steps of delivering a long substrate with application of tensile force, wherein the strength of the tensile force is changed at least between a first delivery step and a second delivery step.
(2) The substrate treatment process of the above item (1), wherein the tensile force is less in the first delivery step than in the second delivery step.
(3) The substrate treatment process of the above item (1) or (2), wherein the extensibility of the substrate in the first step is equal to or higher than that in the second step.
(4) The substrate treatment process of any of the above items (1)-(3), wherein the temperature of the substrate in the first step is controlled to be not lower than 40xc2x0 C.
(5) The substrate treatment process of any of the above items (1)-(4), wherein the tensile force in the first step ranges from 300 to 800 N.
(6) The substrate treatment process of any of the above items (1)-(5), wherein the difference in the tensile strength between the first step and the second step ranges from 50 to 200 N.
(7) The substrate treatment process of any of the above items (1)-(6), wherein the tensile force is applied stepwise (tapered tension) in the first step and/or the second step.
The xe2x80x9cdelivery stepxe2x80x9d in the specification of the present invention means a series of steps from the start to the end of the delivery. For example, in the roll-to-roll system, the operation from winding-off of the long substrate from a roll to winding-up of the substrate by another roll is regarded as one delivery step.